A holey fiber or a photonic-crystal fiber is a new type of an optical fiber which has a core region at the center of the optical fiber and a cladding region formed at the circumference of the core region, which has plurality of holes distributed around the core region. The holes in the cladding lower the average refractive index and by using the principle of total reflection in light, the fiber propagates light in a core region. The holey fiber enables characteristic(s), which is impossible for ordinal optical fibers, by controlling the refractive index using the holes.
At the same time, by using non-linear optical phenomenon in optical fibers, SC (Supercontinuum) light source, which generates a SC light with extremely wide wavelength spectrum, is investigated widely. Development of optical fibers for SC light source is primarily done in a communication wavelength spectrum mainly at 1550 nm. Recently, optical fibers for SC light source in 1050 nm band, which can use Yb doped optical amplifier, and in 850 nm band, which can use fiber lasers, are also started to be investigated.
Optical fibers for SC light source must have its zero-dispersion wavelength in neighborhood of wavelength to be used. However, ordinary silica-base optical fiber has negative material dispersion at the wavelength below 1270 nm and the waveguide dispersion cannot be positive. Therefore, at the wavelength below 1270 nm, the wavelength dispersion cannot be zero. However, by using a holey fiber, the structure can be optimized to have an positive waveguide dispersion and there are studies to create holey fibers which can have zero-dispersion wavelength at 1050 nm, at 850 nm or at the wavelength below those numbers (see, for example, 1) J. C. Knight et al., “Anomalous Dispersion in Photonic Crystal Fiber” IEEE Photonics Technology Letters, vol. 12, p. 807 (2000); 2) CRYSTAL FIBRE A/S “NONLINEAR PHOTONIC CRYSTAL FIBERS SELECTED DATASHEETS 800 NM FIBERS NL-800 list”, [online], [searched on Sep. 26, 2007], internet (URL: http://www.crystal-fibre.com/products/nonlinear.shtm); and 3) J. K. Ranka et al., “Optical properties of high-delta air-silica microstructure optical fiber” Optics Letters, vol. 25, p. 796 (2000).
However, when structure of a holey fiber is optimized to have large positive waveguide dispersion, confinement of light into a core region becomes extremely strong, and, a higher order mode(s) will exist as a propagation mode(s) under the zero-dispersion wavelength, in addition to the fundamental mode. Therefore, the holey fiber becomes a multimode optical fiber.
For example, according to above J. C. Knight et al., “Anomalous Dispersion in Photonic Crystal Fiber” reference, extremely short wavelength holey fiber with zero-dispersion wavelength of 565 nm is reported. However, this holey fiber works as multi-mode at the zero-dispersion wavelength. Also, according to above CRYSTAL FIBRE A/S “NONLINEAR PHOTONIC CRYSTAL FIBERS SELECTED DATASHEETS 800 NM FIBERS NL-800 List” reference, holey fibers with zero-dispersion wavelength of less than 700 nm are reported. However, those holey fibers have cut-off wavelength(s) larger than the zero-dispersion wavelength, and therefore, it works as multi-mode.
Also, according to above J. C. Knight et al., “Anomalous Dispersion in Photonic Crystal Fiber” reference, a holey fiber, which acts as single mode at its zero-dispersion wavelength of 700 nm, is also reported. In addition, according to above CRYSTAL FIBRE A/S “NONLINEAR PHOTONIC CRYSTAL FIBERS SELECTED DATASHEETS 800 NM FIBERS NL-800 List” reference, a holey fiber, which acts as single mode at its zero-dispersion wavelength of 750 nm, is also reported. Furthermore, according to above J. K. Ranka et al., “Optical properties of high-delta air-silica microstructure optical fiber”, a holey fiber with a zero-dispersion of 765 nm is reported. This holey fiber has higher order mode(s) at its zero-dispersion wavelength, but since difference in effective refractive indexes between a fundamental mode and the higher order modes are large, it is reported that it can practically be used as single mode.
If a holey fiber has its zero-dispersion wavelength at visible light spectrum of less than 700 nm and can operate as single mode at its zero-dispersion wavelength, it can be easily used as a visible light spectrum SC light source. For example, it is considered to serve many uses as a light source for various optical sensors. However, ordinal holey fibers with its zero-dispersion wavelength of less than 700 nm operate as multi-mode at its zero-dispersion wavelength. Because of that, mode interference and/or modal dispersion is generated and it causes problem for applications which requires high dispersion control such as SC light source.